Well for a mortal sin to be truly a mortal sin there are three conditions that must take place...1. grave offense, 2. knowledge that it is a grave offense, 3. the consent to commit the sin.

In the CCC it says, that habits, duress, psychological problems, etc; can diminish mortal sins. People who suffer from bipolar disorder, especially in their manic moods are NOT consenting fully. I can still see things being a sin, but venial at best. I think the added worry about if it is mortal or venial can be harmful for some with bipolar Disorder.

"The Church recognizes, for example, that in the practice of masturbation, psychological factors including adolescent immaturity, lack of psychological balance, and even ingrained habit can influence a person's behavior, and this could lessen or even eliminate moral responsibility."